The ability to predict an aircraft's trajectory is useful for several reasons.
Air traffic management (ATM) would benefit from an improved ability to predict an aircraft's trajectory. Air traffic management is responsible for the safe separation of aircraft, a particularly demanding task in congested airspace such as around airports. ATM decision-support tools based on accurate trajectory predictions could allow a greater volume of aircraft to be handled while maintaining safety.
By trajectory, a four-dimensional description of the aircraft's path is meant. The description may be the evolution of the aircraft's state with time, where the state may include the position of the aircraft's centre of mass and other aspects of its motion such as velocity, attitude and weight. This benefit is particularly significant where ATM is operating in and around airports.
As demand for slots at airports increases, ATM is under constant pressure to increase capacity by decreasing separation between aircraft: increased accuracy in predicting aircraft trajectories enables this to be done without compromising safety. Also, greater predictability in aircraft trajectories allows arrival times to be determined more accurately thereby enabling better coordination with ground operations.
In current ATM practice, aircraft must typically fly set routes. For example, when approaching and departing an airport, aircraft are usually requested to fly a STAR (Standard Terminal Arrival Route) and a SID (Standard Instrument Departure), respectively. However, aircraft operators are requesting additional flexibility to fly according to their preferences, so that they can better pursue their business objectives.
Furthermore, there is an increasing pressure on the ATM system to facilitate the reduction of the environmental impact of aircraft operations. As a result of the above, the ATM system requires the capability to predict operator-preferred trajectories as well as trajectories that minimize the impact on the environment, chiefly in terms of noise and emissions. In addition, the ATM system must be able to exchange descriptions of such trajectories with the operators in order to arrive at a coordinated, conflict-free solution to the traffic problem.
The ability to predict an aircraft's trajectory will also be of benefit to the management of autonomous vehicles such as unmanned air vehicles (UAVs), for example in programming flight plans for UAVs as well as in commanding and de-conflicting their trajectories.
In order to predict aircraft trajectory unambiguously, one must solve a set of differential equations that model both aircraft behavior and atmospheric conditions. The computation process requires inputs corresponding to the aircraft intent, as derived from flight intent.
Aircraft intent must be distinguished from flight intent. Flight intent may be thought of as a generalization of the concept of a flight plan, and so will reflect operational constraints and objectives such as intended or required route and operator preferences. Generally, flight intent will not unambiguously define an aircraft's trajectory, as the information it contains need not close all degrees of freedom of the aircraft's motion. Put another way, there are likely to be many aircraft trajectories that would satisfy a given flight intent. Thus, flight intent may be regarded as a basic blueprint for a flight, but that lacks the specific details required to compute unambiguously a trajectory.
For example, the instructions to be followed during a STAR or a SID would correspond to an example of flight intent. In addition, airline preferences may also form an example of flight intent. To determine aircraft intent, instances of flight intent like a SID procedure, the airline's operational preferences and the actual pilot's decision making process must be combined. This is because aircraft intent comprises a structured set of instructions that are used by a trajectory computation infrastructure to provide an unambiguous trajectory. The instructions should include configuration details of the aircraft (e.g. landing gear deployment), and procedures to be followed during maneuvers and normal flight (e.g. track a certain turn radius or hold a given airspeed). These instructions capture the basic commands and guidance modes at the disposal of the pilot and the aircraft's flight management system to direct the operation of the aircraft. Thus, aircraft intent may be thought of as an abstraction of the way in which an aircraft is commanded to behave by the pilot and/or flight management system. Of course, the pilot's decision making process is influenced by required procedures, for example as required to follow a STAR/SID or to comply with airline operational procedures as defined by the flight intent.
Aircraft intent is expressed using a set of parameters presented so as to allow equations of motion to be solved. The theory of formal languages may be used to implement this formulation: an aircraft intent description language provides the set of instructions and the rules that govern the allowable combinations that express the aircraft intent, and so allow a prediction of the aircraft trajectory.